The rapid increase in the number of users of electronic mail and the low cost of distributing electronic messages via the Internet and other electronic communications networks has made marketing via electronic mail ("e-mail") an attractive advertising medium. Consequently, e-mail is now frequently used as the medium for widespread marketing broadcasts of messages to e-mail addresses, commonly known as "spam."
Users of electronic mail, however, frequently are not eager to have their e-mail boxes filled with unsolicited e-mails. Users accessing the Internet through large service companies such as America Online.RTM. (AOL) or Microsoft Network.RTM. (MSN) or large businesses such as IBM.RTM. and General Motors.RTM. are targeted by e-mail marketers. The sending and receiving of unsolicited e-mail messages are increasing problems for both online services and corporations. Online services object to unsolicited mail because it reduces their users' satisfaction of their services, and corporations want to eliminate unsolicited mail because it reduces worker productivity.
There are a number of known methods for filtering unsolicited e-mail. Typically, these methods are designed to block e-mails from particular e-mail addresses that originate unsolicited e-mail. For example, filtering methods used by America On Line.RTM. and Prodigy.RTM. use an exclusion filter that blocks e-mail messages received from addresses that are suspected sources of unsolicited e-mail are blocked. However, this approach is vulnerable to rapid changes in the source of unsolicited e-mail. Furthermore, because courts have ruled that online services can not automatically block e-mail addresses from their members, these services are available only if the user requests them.
Other known e-mail filtering techniques are based upon an inclusion list, such that e-mail received from any source other than one listed in the inclusion list is discarded as junk. However, these methods require the user and/or service provider continually to update the inclusion list manually. If the inclusion list is not updated regularly, the list will quickly become outdated, resulting in exclusion of desired e-mail messages from new sources and continued inclusion of undesired e-mail messages from old sources.